Optical fiber production generally involves drawing a fiber, which usually is composed of silica glass, and then applying protective coating materials to the fiber. The primary layer typically comprises a relatively soft polymeric material that protects the fiber from displacement, and subsequent losses associated therewith. As well, the primary layer helps to absorb forces applied to the coated fiber and prevent their transmission to the fiber core. Typically, a secondary layer comprising a higher modulus polymeric material is applied to the primary layer. The secondary layer maintains high strength and abrasion resistance of the coated optical fiber. Each fiber thus coated must be capable of withstanding, over its entire length, a stress level to which the fiber will be exposed during installation and service. The coating also functions to prevent airborne particles from impinging upon and adhering to the surface of the drawn fiber, which can weaken the fiber and affect its transmission properties. As well, applying coating to the drawn fiber prevents surface abrasions, which can occur as a result of subsequent manufacturing processes as well as handling during installation.
Optical fibers can be coated using a wet coating process which typically involves drawing a fiber through a reservoir of liquid polymer material and then curing the liquid polymer to harden it by exposure to curing radiation, such as, ultra-violet light. In a dual coating process, the coatings are applied in tandem or simultaneously (within the same applicator or die assembly). The tandem arrangement applies a primary coating layer which is then cured, and then a secondary coating layer is applied and cured. In a simultaneous dual coating arrangement, both coats are applied after which they are cured. In both cases, the primary coating is typically a low modulus polymeric material and the second coating is a relatively high modulus polymeric material. For ease of description, only simultaneous dual coating arrangements are discussed.
As would be expected, from the manufacturing standpoint, it is desirable to produce the maximum amount of coated optical fiber for a given period of time, while maintaining quality standards. As such, maximizing draw speeds of the optical fibers is a primary goal of the manufacturing process. As the draw speed, or fiber velocity, is increased, the flow of the primary and the secondary coatings must be increased to maintain the diameters of those coatings. Typically, in a simultaneous dual coating arrangement, the secondary coating flow rate is comparatively easy to adjust, as either the pressure, or the diameter of the secondary die land can be increased with comparatively few tradeoffs. As would be imagined, it is less time consuming and requires less effort to adjust pressure than to adjust the diameter of the secondary die land. Therefore, it is desirable to select a secondary die land of a given diameter that can accommodate a range of draw speeds, and adjust pressure as required for the varying draw speeds. This is possible because centering of the optical fiber within the dual coating apparatus is maintained in the portion of the apparatus that applies the primary coating. Centering of the optical fiber within the primary coating portion of the dual coating apparatus is accomplished by utilizing the viscoelastic forces created by the polymeric primary coating as it passes through the primary coating land. These viscoelastic forces are maximized by maintaining the diameter of the primary land as small as possible while still maintaining the desired primary coating diameter.
A number of factors affect the primary coating diameter through the primary land, those being the pressure at which the primary coating is applied, the diameter of the primary coating land, the temperature of the optical fiber, the temperature of the primary coating, and the speed at which the fiber is drawn through the land. Adjusting any of these factors, either alone or in combination, creates drawbacks from a manufacturing standpoint. For example, increasing the application pressure of the primary coating tends to reduce centering forces. As well, adjusting the pressure of the primary coating is largely ineffective for increasing the primary coating flow rate, as the flow rate tends to be controlled by drag force due to the relatively small diameter of the primary coating land. Increasing the diameter of the primary land increases the flow rate of the primary coating. However, as the primary land diameter is increased, the viscoelastic forces created within the primary land are reduced, therefore reducing the forces used to center the optical fiber within the apparatus. Reduced centering forces often lead to non-concentric coatings and coating defects. To increase the drag force between the optical fiber and the primary coating, either the coating temperature or the fiber temperature may be reduced. However, reducing the temperatures of the optical fiber and the primary coating requires the increased use of helium (frequently used for cooling). Greater helium consumption translates into increased manufacturing costs, as helium is expensive.
Another problem common to existing dual coating apparatus is that as the draw speed of the optical fiber is increased the ramp scrap, or unusable fiber, is increased. This is largely due to the fact that existing primary coating lands are sized such that the desired primary coating diameter will be achieved for the given optical fiber velocity. The other factors being constant (coating pressure, fiber temperature, coating temperature, etc.), typical primary coating lands can only accommodate a narrow range of draw speeds, such as 15% of the design speed. For example, the primary coating land sized for an optical fiber draw speed of 25 meter/second (m/sec) may only produce good fiber that is drawn between 21 and 25 m/sec. Optical fiber drawn at speeds outside of this narrow range will potentially have primary coatings that are not within acceptable limits. A greater range would require additional helium consumption, as noted above, to reduce the optical fiber temperature and therefore increase the primary coating flow.
From the foregoing, it can be appreciated that it would be desirable to have a simple, less expensive, and effective apparatus for coating optical fibers, thereby facilitating the production of optical fiber at increased draw speeds while maintaining the desired primary coating diameters.